The present invention relates to apparatus which simulates the operation of a stepping motor and, more particularly, to an improved shaft encoder and control feedback circuit which can be employed with a servomotor to simulate the operation of a stepping motor.
The stepping motor has, in recent years, become a basic building block of computer controlled sytems. Modern high performance systems, however, have increased the peformance requirements of stepping motors in terms of higher speeds, higher accelerations, faster settling times, controlled response, higher efficiency, and elimination of resonance phenomena. With conventional stepping motors, some of these performance requirements can be attained only at great expense. Others cannot be attained at all.
Heretofore, optical shaft encoders have been predominantly of two basic types. In one, an opaque disk having transparent regions moves between a light source and a detector. In the other, two such disks are provided, as is a technique of determining average light flux transmitted through the disk pair at predetermined fixed locations with respect to the disk axes. In attempting to apply either type encoder to the problem of simulating a stepping motor, both are found wanting. The former generates a discontinuous feedback signal (e.g., a digital pulse train or binary code), since it inherently includes a deadband of one pulse either side of the desired null. This inherent defect is particularly troublesome near the null location (i.e., the rotational orientation of the disk which should define of the next step of the simulated stepper). The alternative encoders, employing paired disks having unequal slots or displaced axes, are limited in the number of steps which can be defined in a single revolution. Furthermore, when few steps are desired within a revolution, the averaging of light to adjacent slots, and thus the intended direction of rotation, may become indeterminate.
In view of the foregoing, it is a principal object of the present invention to provide apparatus which simulates the operation of a stepping motor and which can achieve better performance characteristics than conventional stepping motors.